Changed minversion in tox to 3.18.0

The patch bumps min version of tox to 3.18.0 in order to
replace tox's whitelist_externals by allowlist_externals option:
https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23

Change-Id: Ied7fec3d6bbb37b42be353f5e4b9aad8c5921d0a

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1
+minversion = 3.18.0
 envlist = pep8,py3,docs,api-ref
 skipsdist = True
 ignore_basepython_conflict = True
@@ -29,7 +29,7 @@ commands = {posargs}
 [testenv:docs]
 # This environment is called from CI scripts to test and publish
 # the API Ref and Docs to docs.openstack.org.
-whitelist_externals = rm
+allowlist_externals = rm
 deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/doc/requirements.txt
 commands =
@@ -39,7 +39,7 @@ commands =
 [testenv:api-ref]
 # This environment is called from CI scripts to test and publish
 # the API Ref to docs.openstack.org.
-whitelist_externals = rm
+allowlist_externals = rm
 deps = {[testenv:docs]deps}
 commands =
   rm -rf api-ref/build